[Hexagon] Early-if-convert branches that may exit the loop

Merge the tail block into the loop in cases where the main loop body
exits early, subject to profitability constraints. This will coalesce
the loop body into fewer blocks.

For example:
  loop:                           loop:
    // loop body                      // loop body
    if (...) jump exit      -->       // more body
  more:                               if (...) jump exit
    // more body                      jump loop
    jump loop

llvm-svn: 297033